Fluorine reactes with $KHSO_4$ to form HF and $\underline X$. Which one of the following is $\underline X$?

$\begin {array} {1 1} (1)\;SO_3 & \quad (2)\;K_2SO_4 \\ (3)\;K_2S_2O_8 & \quad (4)\;H_2S \end {array}$

(3) $K_2S_2O_8$